Telomeres are higher order nucleoprotein structures that physically cap the chromosome terminus and help to perserve genome integrity. In cells with unlimited proliferative capacity, including 95 percent of human cancers, telomeres are maintained by telomerase. By contrast, telomerase is absent from most cells of the normal soma and telomeres inexorably shorten until chromosome ends become uncapped and indistinguishable from double strand breaks. End-to-end fusions are induced, ultimately leading to cell cycle arrest. Here we propose to exploit Arab idopsis to elucidate the structure and function of the telomere cap to better understand how this device provides stability to the genome, and facilitates continued cell proliferation. Our preliminary results indicate that Arabidopsis has an exceptionally high tolerance for telomere dysfunction. This finding, coupled with the fascile genetics of Arabidopsis, its completely sequenced genome, and arsenal of transgenic tools demonstrate that this system can offer unique opportunities for investigating essential genes in telomere biology. The first two goals of the proposal exploit the telomerase-deficient Arabidopsis model we developed to probe the molecular transition between capped and uncapped chromosomes. In Aim I, we will examine the frequency of telomere recombination in cells with shortened, but functional telomeres. We will also determine the contribution of somatic and meiotic recombination to telomere maintenance in the absence of telomerase. In Aim 2, we will explore the mechanism of telomere fusions by sequencing DNA at chromosome junctions, and by assessing the role of the non-homologous end-joining pathway in this process. A genetic strategy is also proposed to examine the propagation of cells harboring chromosome fusions. The last two aims focus on proteins that form the chromosome cap. In Aim 3, we will examine the function of two putative Pot1 orthologs in Arabidopsis by elucidating their DNA binding properties and molecular partners, and by determining the consequences of over-expression and gene disruption. In Aim 4, biochemical and genetic approaches are described to discover genes that facilitate chromosome capping. Because virtually every mammalian gene involved in telomere homeostasis has an Arabidopsis counterpart, these experiments should uncover mechanisms common to all multicellular organisms.